The present invention relates to a rubber composition for coating steel cords improved in the adhesion to steel cords, and a rubber product reinforced with steel cords coated with the rubber composition.
Steel cords are widely used as the reinforcement for rubber products, such as pneumatic tires and conveyor belts, by virtue of their far superiority in overall performance as the reinforcement as composed to reinforcing cords made of other fiber materials.
In the conventional steel cords used for these pneumatic tires and conveyor belts, the steel cords side was subjected to brass plating, galvanizing, etc., for the purpose of enhancing its adhesion to the rubber, and various proposals have been made on the composition and thickness of the plating. On the other hand, an adhesion accelerator, for enhancing the adhesion of the steel cords to the coating rubber is used on the side of the rubber composition for coating the steel cords. Typical examples of the adhesion accelerator include cobalt salts of natural acids, such as cobalt naphthenate, cobalt rosinate and cobalt stearate, and cobalt salts of synthetic acids, such as cobalt neodecanoate, cobalt octylate and cobalt propionate.
Although the rubber compositions containing these cobalt salts of organic acids exhibit a good initial adhesion to steel cords, they are disadvantageous in that the adhesion becomes remarkably lower during use or storage under high-temperature and high-humidity conditions.
In order to solve this drawback, for example, (U.S. Pat. No. 4,234,496) proposes an adhesion accelerator comprising a complex of a carboxylic acid having 7 to 11 carbon atoms with boron, and U.S. Pat. No. 4,588,766 proposes an adhesion accelerator comprising a product of a reaction of a carboxylic acid salt of cobalt or nickel with a boric acid salt of an alkaline earth metal. These adhesion accelerators, however, have drawbacks that they lower the initial adhesion to the steel cords and further deteriorate rubber properties.
Further, U.S. Pat. No. 4,609,499 proposes a rubber composition containing a reaction product comprising a monocarboxylic acid, cobalt and boron, and U.S. Pat. No. 4,513,123 proposes a rubber composition containing an organic cobalt compound, sulfur and dithiodipropionic acid. However, the former is unsatisfactory in its waterproof adhesion when it is wet, while in the latter, the adhesion, which is very important for the production of reinforced rubber products, greatly depends upon vulcanization conditions, such as vulcanization temperature and time.
Further, Japanese patent application Kokai publication No. 1-153783 discloses a metallic soap comprising a cobalt salt of an organic carboxylic acid and boron, which has a low content of water-extractable boron. Although this soap has an improved wet heat resistance, it does not satisfy the practical performance requirement due to the great dependency of the adhesion upon the vulcanization conditions.